The present invention relates to a rubber composition which can reduce rolling resistance.
Petroleum oils such as aromatic oil have been used as softening agent in a rubber composition for a tire. However, there has been a problem that rolling resistance is increased and fuel efficiency is lowered when petroleum oils are blended. In recent years, due to the serious interest in the global environmental problems, demand for new softening agent alternative to petroleum oils is growing.
An object of the present invention is to provide a novel rubber composition which can reduce rolling resistance, decreasing the amount of petroleum oils to be used.
Examining the solution, it has been found that rolling resistance can be reduced by using particular vegetable oils instead of petroleum oil, and the present invention has been completed.
That is, the present invention relates to a rubber composition comprising 5 to 150 parts by weight of an inorganic filler, 0 to 30 parts by weight of a silane coupling agent and 5 to 100 parts by weight of a vegetable oil having an iodine value of at most 130, based on 100 parts by weight of a diene rubber.
It is preferable to use a natural rubber as the diene rubber in the rubber composition.
The rubber composition of the present invention comprises a diene rubber, an inorganic filler and a vegetable oil.
Examples of diene rubber are synthetic rubbers such as styrene-butadiene rubber (SBR), butadiene rubber (BR) and butyl rubber (IIR); and natural rubbers. Such diene rubbers may be used alone or in combination of two or more. Natural rubbers are preferable from the viewpoint of reducing rolling resistance. The amount of natural rubber is preferably at least 75% by weight, more preferably at least 85% by weight based on the diene rubber. When the amount of natural rubber is less than 75% by weight, reduction of rolling resistance tends to be small.
The inorganic filler is used as an alternative to carbon black which has been often used as a reinforcing filler. By reducing the amount of carbon black with using the inorganic filler alternatively, rolling resistance can be reduced.
Examples of inorganic fillers are silica, sericite, calcium carbonate, clay, alumina, aluminum hydroxide, magnesium hydroxide, magnesium oxide and titanium oxide. Among them, silica is preferable to ensure the reinforcement of the rubber.
When silica is used, it is preferable that silica has a BET specific surface area of 150 to 250 m2/g. When the BET specific surface area of silica is less than 150 m2/g, reinforcing property tends to be inferior. When the BET specific surface area of silica is more than 250 m2/g, there is a tendency that dispersibility is inferior, agglomeration is caused and thus physical properties are decreased.
The amount of inorganic filler is 5 to 150 parts by weight based on 100 parts of the diene rubber. When the amount of inorganic filler is less than 5 parts by weight, reinforcing property tends to be insufficient. When the amount of inorganic filler is more than 150 parts by weight, processability tends to be inferior. The lower limit of the amount of inorganic filler is preferably 30 parts by weight, more preferably 40 parts by weight. The upper limit of the amount of inorganic filler is preferably 120 parts by weight, more preferably 100 parts by weight.
The amount of inorganic filler is at least 75% by weight, preferably at least 85% by weight based on the total amount of fillers. When the amount of inorganic filler is less than 75% by weight, reduction of rolling resistance tends to be small.
It is preferable to use a silane coupling agent together with the inorganic filler. There is no particular limit for the kind of silane coupling agent as long as it is currently used in the field of production of tires. Examples of silane coupling agent are bis(3-triethoxysilylpropyl)tetrasulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, bis(2-triethoxysilylpropyl)tetrasulfide, 3-mercaptopropyltriethoxysilane and 2-mercaptoethyltrimethoxysilane. These silane coupling agents may be used alone or in any combination. Among these, bis(3-triethoxysilylpropyl)tetrasulfide and 3-mercaptopropyltriethoxysilane are preferable from the viewpoint of reinforcing property of silane coupling agent and the resulting processability. Furthermore, bis(3-triethoxysilylpropyl)tetrasulfide is particularly preferable from the viewpoint of processability.
It is preferable that the amount of silane coupling agent is 0 to 30 parts by weight based on the 100 parts of the diene rubber.
It is also preferable that the amount of silane coupling agent is 3 to 20% by weight based on inorganic fillers. When the amount of silane coupling agent is less than 3% by weight, the effect of adding silane coupling agent is insufficient. When the amount of silane coupling agent is more than 20% by weight, the effect to be obtained remains small in spite of cost increase.
As to the vegetable oil, those having a low unsaturation degree are preferable. For example, semi-drying oils having an iodine number of 100 to 130, non-drying oils and solid oils having an iodine number of at most 100 are preferable. Herein, the iodine number is defined as the amount of iodine based on gram, which can be absorbed by 100 g of oil. Concrete examples of these vegetable oils are semi-drying oils such as cotton seed oil and rape seed oil; non-drying oils such as castor oil and arachis oil; and solid oils such as palm oil and cocoanut oil. When the iodine number of vegetable oil is more than 130, there is a tendency that tan 8 is increased and hardness is lowered, resulting in increase of rolling resistance and decrease of steering stability. More preferable upper limit of the iodine number of the vegetable oil is 120.
The amount of vegetable oil is 5 to 100 parts by weight based on 100 parts of the diene rubber. When the amount of vegetable oil is less than 5 parts by weight, softening effect on rubber tends to be insufficient. When the amount of vegetable oil is more than 100 parts by weight, processability tends to decrease. Preferable upper limit of the amount of vegetable oil is 80 parts by weight.
Further, it is preferable that the vegetable oil accounts for at least 75% by weight, particularly at least 85% by weight of the total oil. When the amount of vegetable oil is less than 75% by weight, lowering effect of rolling resistance tends to be inferior.
It is preferable to blend vegetable oil so that the hardness of rubber becomes 40 to 90 after vulcanization. When the hardness of rubber is less than 40, there is a tendency that required rigidity cannot be achieved. When the hardness of rubber is more than 90, processability tends to be inferior.
The rubber composition of the present invention may be incorporated with wax, antioxidant, cured resin, adhesive, stearic acid, zinc oxide, vulcanization accelerator, sulfur and the like, in addition to diene rubber, inorganic filler, silane coupling agent and vegetable oil.
The rubber composition of the present invention is obtained by kneading the above components and can be used as tire parts by molding into tread side wall, case, inner liner, breaker and bead. In particular, it is preferable to use the rubber composition of the present invention for tread from the viewpoint that the tread contributes largely to the reduction of rolling resistance.
When preparing a case, natural fibers such as rayon and acetate made from wood pulp or cupra made from cotton seed crude linter may be useful instead of commonly used synthetic fibers. Among them, rayon is preferable because rayon has a high strength required for tires.